1. Field of the Invention
The subject matter disclosed generally relates to the field of hard disk drives.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. There have been developed magnetic heads that have a write element for magnetizing the disks and a separate read element for sensing the magnetic fields of the disks. The read element is typically constructed from a magneto-resistive material. The magneto-resistive material has a resistance that varies with the magnetic fields of the disk. Heads with magneto-resistive read elements are commonly referred to as magneto-resistive (MR) heads.
Each head is attached to a flexure beam to create an subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are attached to an actuator arm that has a voice coil coupled to a magnet assembly. The voice coil and magnet assembly create a voice coil motor that can pivot the actuator arm and move the heads across the disks.
Information is typically stored within annular tracks that extend across each surface of a disk. The voice coil motor can move the heads to different track locations to access data stored onto the disk surfaces. Each track is typically divided into a plurality of adjacent sectors. Each sector may have one or more data fields. Each data field has a series of magnetic transitions that are decoded into binary data. The spacing between transitions define the bit density of the disk drive. It is generally desirable to provide a high bit density to increase the overall storage capacity of the drive.
The disk drive can be subjected to shock and vibration loads that cause undesirable movement of the heads. Most disk drives include an internal servo routine that compensates for such undesirable movement. A typical servo routine will utilize information from servo bits on a data track to determine a position error signal (PES). The PES is used to drive the voice coil motor and maintain the heads on the centerlines of the tracks.
The voice coil motor has a relatively slow reaction time between when the disk drive controller provides an output signal and when the head actually moves in response to the signal. This slow reaction time degrades the track following and seek settling times of the disk drive. The slow reaction time is particularly burdensome when the servo routine is attempting to compensate for high frequency vibration.
The servo routine must wait for the reading of servo bits on the data track to receive feedback information regarding the location of the head. For embedded servo disk drives the servo bits are placed at periodic intervals throughout a track. This means that there is not a continuous stream of servo data. Additionally, the servo bits only provide positional information. Positional information does not provide feedback on the actual vibration/shock forces applied to the actuator arm.
There have been proposals to integrate a piezoelectric element into the head gimbal assembly to improve the reaction time of head movement. Alternatively, there has been proposed integrating a MEMS into the head to also achieve fast reaction times. To date, these solutions are impractical for mass producing hard disk drives.